The present invention relates to a fluid pressure controlling device for hydraulic brake system of automobiles or the like vehicles. More particularly, the invention is concerned with an improvement in fluid pressure controlling device for vehicle brake system of a type having a valve housing provided with an inlet chamber communicating with the output port of a master cylinder and an output chamber communicating with a rear wheel brake; a resilient valve seat separating the input and output chambers from each other and having a valve port intercommunicating with the input and output chambers; a valve member including a valve rod slidably received in the valve housing and extending through the valve port and a pressure-receiving piston formed integrally with the valve rod and received in the output chamber so as to open and close the valve port, and a spring adapted to bias the valve member in such a direction that the pressure-receiving piston is moved away from the resilient valve port.
In this type of fluid pressure control device, when a hydraulic pressure exceeding a predetermined pressure is generated in the master cylinder during the braking, the valve member and the resilient valve seat cooperate to open and close the valve port thereby to reduce the pressure in the master cylinder proportionally and transmit the reduced pressure to the wheel brake.
In the conventional device of the kind described, the valve member and the resilient valve seat are designed and constructed to make a surface contact with each other. Therefore, the sufficient sealing effect is often failed if one of the valve seat and the valve member has a deteriorated flatness or if the valve seat and the valve member is inclined to each other, resulting in a deteriorated braking function. This problem is observed most often in the case where the resilient valve seat is made by molding.
In the normal braking operation, the valve member is displaced by resilient deflection of the valve seat while maintaining contact with the latter, in accordance with the decrease of the pressure in the master cylinder, i.e. in the input chamber, thereby to reduce the pressure in the wheel brake, i.e. in the output chamber. As a result, the valve member is moved in the reverse direction by the sum of the force of residual pressure in the master cylinder and the reactional force of the spring, thereby to open the valve port. This operation is repeated to reduce the fluid pressure in the wheel brake in accordance with the reduction of fluid pressure in the master cylinder.
Hitherto, the deflecting part of the resilient valve seat including the seat surface for contacting the valve member is formed to have a uniform and comparatively large thickness, so that the deflection of the valve seat is less liable to occur when the braking operation is ceased, so that the response of valve member to the decrease of pressure in the master cylinder is not so good. In consequence, the ratio of pressure in the brake to the residual pressure in the master cylinder during the brake-releasing period is largely differed from that during the braking period to cause a hysteresis in the braking.